I would cut it in half based on the # of yds you bought and then piece a horizontal piecing that would make up 6-12 inches in width by 69+ 12 inches. Sew 39inches by 3 1/2 yards to the horizontal strip and then to the other 3 1/2 yards (actually you just need it to be about 79+12 inches = 91 inches or a little more than 2 1/2 yards each) so that the pieced part is in the middle of the backing. Then quilt as normal.
I do this often with my throw quilts that are around 80 inches on the long side.....works like a charm and I don't have to piece the top part or the bottom. And it looks like I did it on purpose and brings the front around to the back!